The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Piezoelectric devices are often used in small, compact, light-weight motor applications to provide mechanical energy that can be used to generate mechanical work. Generally, a voltage is applied to a structure, e.g., a beam, comprising a piezoelectric material that causes the piezoelectric structure to flex or bow. For example, a piezoelectric beam can comprise piezoelectric layers formed on opposite sides of a flexible supporting substrate. In such instances, the voltage is applied across one of the piezoelectric layers causing this first layer to elongate, while substantially simultaneously, a reverse voltage is applied across the other piezoelectric layer causing the second layer to shorten. Thus, the beam is caused to bow or flex, resulting in a physical displacement of at least a portion of the beam. This displacement can be utilized to provide mechanical work. For example, the polarity of the applied voltages can be cyclically alternated such that the portion of the piezoelectric beam that is displaced oscillates between displacement in a first direction and displacement in an opposite second direction. This oscillating displacement can be utilized to provide mechanical energy, or work. For example, the oscillating displacement can be used to drive a piston of a pneumatic device.
However, for the typical piezoelectric motor device to work efficiently, the cyclic frequency of the alternating voltage must occur at or above the resonance frequency of the piezoelectric beam that is determined by the material characteristics of the beam. That is, the stiffness of the beam, including the piezoelectric material, and beam length determine how quickly and how much the beam will flex when voltage is applied. Oscillation of the applied voltage, and therefore oscillation of the beam, at frequency below the resonance frequency will produce very little displacement of the piezoelectric beam. Conversely, voltage and beam oscillation at or above the resonance frequency will provide maximum displacement. However, the material characteristics of the beam typically require that a considerable amount of electrical energy be provided to overcome the stiffness of the beam and cause the beam to flex. This typically limits the oscillation frequency at which the beam can efficiently provide mechanical energy to a very narrow bandwidth.
Accordingly, there exists a need for a piezoelectric motor that is capable of efficient operation at beam oscillation frequencies above and below the resonance frequency of the piezoelectric beam.